The present invention concerns a web dryer operating according to the air floating principle, comprising a plurality of nozzle members by which air, or an equivalent gaseous fluid, is blown into contiguity with the web that has to be dried, and which is at the same time supported without mechanical contact, and said nozzle members having a supporting surface producing said supporting effect.
Means which are based on the blowing of gas and operate according to the air float principle are employed on paper manufacturing and processing machines for contactless cleaning, drying and stabilizing of the web. In the operations mentioned, the gas is introduced with the aid of various nozzle means on one or both sides of the web under treatment, the gas being thereafter drawn off before the next nozzle for reuse.
The dryers known in prior art for contactless treatment of the web (float dryers) are composed of a plurality of nozzles from which are directed against the web a gas flow supporting and drying the web. The nozzles of the prior art used in said dryers can be divided into two groups: overpressure nozzles and nozzles with subatmospheric pressure. The operation of the former type is based on the so-called air cushion principle, wherein the air jet causes a static overpressure in the space between the nozzle and the web. The group of subatmospheric pressure nozzles includes the so-called airfoil nozzles, which attract the web and stabilize the running of the web. The attraction acting on the web is well known to derive from a flow field of the gas parallelling the web, causing a static under-pressure between the web and the supporting surface of the nozzle, or the so-called carrying surface. It is frequent that the so-called Coanda phenomenon is employed in overpressure as well as subatmospheric pressure nozzles to guide the air in desired direction.
The overpressure nozzles in float dryers of the prior art direct sharp air jets substantially against the web. Such a localized impingement of the air jet on the web significantly enhances the heat transfer at the point where the jet and the web meet, thus giving rise to non-uniform distribution of the heat transfer coefficient in the longitudinal direction of the web, and this may cause quality defects in the web that is being treated. A further detriment when overpressure nozzles are being used is that because of the over-pressure feature they may not be used in one-sided treatment of the web.
Regarding the patent literature associated with the present invention, reference is made to the following patents: U.S. Pat. No. 3,711,960, Finnish Pat. No. 42 522 and German publicizing print No. 2 020 430.
The design of the dryers known through the said U.S. Pat. No. 3,711,960 and the German publicizing print No. 2 020 430 and of their subatmospheric pressure nozzles, is characterized by the feature that the nozzle slit opening on the side of the entry margin of the nozzle's carrying surface is extended onto a curved flow guiding surface connecting to the front margin of the carrying surface, so that the flow can be made to follow the carrying surface. These dryers of the prior art present the drawback that the blowing action parallel to the web tends to eject drying gas that has already been cooled in the preceding suction space, thereby lowering the differential temperature between the web and the drying gas and as a consequence reducing the heat transfer capacity. In dryers known in the art, the distance of the web from the carrying surface will be quite small (2-3 mm), which imposes high requirements on the smoothness and straightness of the drying surface (the carrying surface). This implies major requirements to be imposed on the design in the manufacturing of great width (over 3 m) dryers spanning the whole web.
Through the above-mentioned Finnish Pat. No. 42 522 a dryer is known by the nozzles of which the air is blown on one side of the web in the form of jets parallelling it and which in the breadth direction of the web give rise to discontinuities, for which reason a non-uniform heat transfer capacity is experienced. For the same reason the web stability is also poor, and this dryer cannot be used to handle thin webs, owing to the fluttering which is produced in their case. It is also impossible in this nozzle to employ high blowing rates, and the nozzle is not usable in two-sided web treatment. In connection with modern, high output paper manufacturing and conversion machines the dryers considered will be bulky, space-consuming and expensive. Owing to the low stabilizing power of the carrying surface in dryers of prior art, it is further noted that the one-sided treatment of heavy material webs has previously only been possible in the horizontal plane, with blowing from underneath the web. This fact has tended to restrict the designing of the dryer and to increase the apparatus size.